Heretofore, an actinic ray curable ink which is cured with an active energy radiation such as an ultraviolet ray and an electron beam, is put into practical use in various applications such as coating materials for plastics, paper sheets, woodworks and inorganic materials; adhesive agents; printing inks; print circuit boards; and electric insulations.
As the polymerizable ink, there is an ultraviolet ray curable inkjet ink which is cured by ultraviolet ray radiation, is well known. In recent years, attention is being paid to the inkjet recording method using the ultraviolet ray curable inkjet ink from the viewpoints that the ink is rapidly dried and the method enables recording (printing) onto a recording medium without ink absorption property.
However, in the recording (printing) method which uses the ultraviolet ray curable inkjet ink, coalescence of adjacent dots, which is a problem during high-speed recording (for example, with a transportation speed of a recording medium of 15 m/s or more in a line recording method or with a printing speed of 20 m2/hour in a serial (shuttle) recording method) cannot be fully prevented. As a result, there occurs a problem of deterioration of image quality in an image formed by printing.
As a method to solve such a problem, there is a method in which an image is recorded on a recording medium through an inkjet process, employing an ink which is in a solid state at ordinary temperature and heated to a liquid state (see Patent Document 1). However, even when a radiation curable ink is employed which is in a solid state at ordinary temperature and heated to a liquid state, there is still problem in that a satisfactory image quality is not obtained.
As an ink containing a gelling agent, there is known an ink for three dimension inkjet printing, for example, one containing a polymerizable compound and wax which functions as a gelling agent (see Patent Document 2). Further, it is known that an ultraviolet ray curable urethane acrylate resin with a relatively high molecular weight is used as the polymerizable compound in order to form a three dimension structure.
However, an inkjet ink for the three dimension inkjet printing has, in the so-called high speed printing forming a plane image at high speed as described above, problem in that ink ejection properties are poor and ink supplied to a recording medium sterically solidifies, resulting in poor plane image. Therefore, the inkjet ink itself cannot be applied.
Further, when supplying the inkjet ink containing a gelling agent onto a recording medium, converting the ink to a gel state on the recording medium, and exposing the gelled ink to actinic rays to polymerize a polymerizable compound, there occurs the phenomenon that the recording medium curls on account of shrinkage (cure shrinkage) of image portions due to the polymerization. Since volume shrinkage (reduction) occurs by action of the gelling agent during gelling, the degree of this cure shrinkage of the inkjet ink containing a gelling agent is large as compared with that of a conventional actinic ray curable inkjet ink which does not contain a gelling agent.